Merge pull request #1712 from BartSX/iar

[STM32L4] Fix IAR section placement failed error

Author: 0xc0170

libraries/mbed/targets/cmsis/TARGET_STM/TARGET_STM32L4/TARGET_DISCO_L476VG/TOOLCHAIN_IAR/stm32l476xx.icf

@@ -31,5 +31,5 @@ do not initialize  { section .noinit };
 place at address mem:__intvec_start__ { readonly section .intvec };
 
 place in ROM_region   { readonly };
-place in SRAM2_region   { readwrite, block STACKHEAP };
-place in SRAM1_region { };
+place in SRAM2_region   { readwrite };
+place in SRAM1_region { block STACKHEAP };